This invention relates to a method and apparatus for simultaneously determining the thickness of multiple thin films (e.g., metal films) contained in a multilayer structure (e.g., a microelectronic device).
During fabrication of microelectronic devices, thin films of metals and metal alloys are deposited on silicon wafers and used as electrical conductors, adhesion-promoting layers, and diffusion barriers. Microprocessors, for example, use metal films of copper, tungsten, and aluminum as electrical conductors and interconnects; titanium and tantalum as adhesion-promoting layers; and titanium:nitride and tantalum:nitride as diffusion barriers. Thickness variations in these films can modify their electrical and mechanical properties, thereby affecting the performance of the microprocessor. The target thickness values of metal films vary depending on their function: conductors and interconnects are typically 3000-10000 angstroms thick, while adhesion-promoting and diffusion-barrier layers are typically between 100-500 angstroms thick.
Metal films are typically deposited and patterned in complex geometries in the microprocessor. A geometry currently used in microelectronics fabrication is a "damascene" or "dual damascene" structure. Damascene-type structures, used primarily to form copper conductors and interconnects, are typically formed by a multi-step process: i) an oxide layer on a wafer is first etched to have a series of trenches and then coated with a diffusion-barrier layer of tantalum or tantalum nitride; ii) copper is electrolytically plated onto the wafer to fill the coated trenches; iii) the structure is then mechanically polished to remove excess copper, leaving only trenches filled with the diffusion-barrier layer and copper. The resulting structure is a series of separated copper lines having a thickness of a few thousand angstroms, a width and periodicity of about 0.5 microns, and a length of several millimeters.
During typical fabrication processes, films are deposited to have a thickness of within a few percent (e.g., 5-100 angstroms, a value roughly equivalent to one or two seconds of human fingernail growth) of their target value. Because of these rigid tolerances, film thickness is often measured as a quality-control parameter during and/or after the microprocessor's fabrication. Noncontact, nondestructive measurement techniques (e.g., optical techniques) are preferred because they can measured patterned "product" samples, (e.g., damascene samples) rather than "monitor" samples. Measurement of product samples accurately indicates errors in fabrication processes and additionally reduces costs associated with monitor samples.
One optical technique for film-thickness measurements uses a single, short (typically 100.times.10.sup.-15 seconds, i.e. 100 fs) optical pulse to generate an acoustic pulse that propagates into a multilayer structure. The acoustic pulse reflects off various interfaces (i.e., layer/layer and substrate/layer interfaces) in the structure, thus causing it to return to the structure's surface. The returning pulse modulates the surface reflectivity and is measured with a variably delayed optical probe pulse. The thickness of the layers in the structure is determined by analyzing the time dependence of the reflected probe beam and the sound velocities of the acoustic pulse.
A related method splits a single short optical pulse into two spatially separate pulses using a partially reflecting mirror (e.g., a beam-splitter). A lens collects and overlaps the two optical pulses on a structure's surface to form an interference pattern containing periodic "light" (constructive interference) and "dark" (destructive interference) regions. The sample absorbs light in each of the light regions to generate an acoustic wave that includes a component that propagates into the structure and reflects off the various interfaces. A probe beam diffracts off the reflected acoustic waves that return to the surface to form a signal beam that is analyzed as described above.